A class of compounds commonly referred to as liquid crystals have been extensively exploited as display devices, particularly in electronic calculators. Over a given temperature range such compounds exhibit rheological properties similar to fluids and optical properties similar to crystals. This condition has been referred to as a separate state of matter called the mesomorphic state or mesophase. If a source of white light is directed on the surface of a thin layer of one of these crystals the scattering properties are such that only a narrow band of frequencies (colors) are reflected from its surface. The center frequency of this band is directly proportional to temperature. The color changes are most easily observed when the back of the thin crystal is coated with a black or light absorbing layer.
The compounds themselves are almost exclusively derivatives of cholesteral or cholestryl esters. Some of these are described in more detail in U.S. Pat. No. 3,620,889 entitled "Liquid Crystal System" granted to Donald H. Baltzer, Nov. 16, 1971. The patent also teaches how to form mixtures of these compounds, so that the temperature range of operation can be centered at various values including normal room temperature (about 25.degree. C). The patent also discloses the use of carbon particles in the crystal to improve color visibility in situations where the use of a black background is not feasible.
It has been suggested that such a crystal be used to detect infrared (thermal) images. To be effective in this role its temperature would have to be held constant to a small fraction of a degree in spite of ambient temperature changes and the radiation load that the crystal would carry. Systems to do this tend to be expensive and bulky. To compete with existing solid state systems a unique approach to these problems is needed.